P415: A COMPLEX INTERPLAY OF INTRA- AND EXTRACELLULAR FACTORS REGULATES THE OUTCOME OF FETAL- AND ADULT-DERIVED MLL-REARRANGED LEUKEMIA
Authors: Sudip Ghosh, Maria Jassinskaja, Mina Davoudi, Melina Claesson Stern, Ugarit Daher, Mohamed Eldeeb, David Bryder, Jenny Hansson
Journal: HemaSphere (2023)

Background: Despite decades of research, infant and adult MLL-rearranged (MLLr) acute leukaemia remains a disease with few treatment options and a dismal prognosis. The majority of infant leukaemias are driven by in utero-acquired MLL rearrangements with no or few cooperating mutations, demonstrating the exceedingly high penetrance of the mutation and aggressiveness of the disease. In adults, however, MLLr leukaemias represent less than 10% of all acute leukaemia cases, and rarely present as a de novo disease. In addition to the striking difference in prevalence of MLL translocations in in utero-initiated leukaemia relative to disease acquired later in life, a marked discrepancy exists in the occurrence of different disease phenotypes between infants and adults; MLLr leukaemia predominately manifests as acute lymphoblastic leukaemia (ALL) in infants, while acute myeloid leukaemia (AML) presents at similar frequency as ALL in MLLr leukaemia in adults. Aims: Here, we aim to delineate the changes in cellular function and molecular mechanisms governing initiation and progression of MLLr leukaemia in infants and adults to facilitate development of preventative strategies and age-tailored therapies. Methods: Using an inducible mouse model of MLL-ENL-mediated leukemogenesis, we have performed thorough in vitro and in vivo assays in combination with highly multiplexed quantitative mass spectrometry (MS)-based proteomics to functionally and molecularly characterise foetal- and adult-origin MLLr leukaemia. In addition, we have used data-independent acquisition MS to compare the extracellular proteome of the foetal liver and adult bone marrow. Results: We reveal that foetal MLL-ENL-expressing lymphomyeloid multipotent progenitors (LMPPs) are intrinsically programmed towards a lymphoid fate but give rise to myeloid leukaemia in vivo, highlighting a complex interplay of intracellular and niche-associated factors in determining disease subtype. Using MS-based quantitative proteomics, we characterize the early proteomic events of MLL-ENL-mediated leukaemic transformation in foetal and adult haematopoietic progenitors and reveal that differentiation arrest and metabolic rewiring are early events in MLLr leukemia regardless of developmental origin. We show that whereas adult leukemic protein signatures are mainly downregulation of ribosomal and metabolic proteins, MLLr leukemia initiation in fetal hematopoietic progenitor cells is governed by an enrichment of translation-associated and HDAC signaling proteins, and a decreased expression of proteins involved in apoptosis, indicating a differential vulnerability of fetal and adult leukemic cells to modulation of these processes. In line with the disparity in incidence of ALL and AML in infants versus adults, foetal cells retain a more lymphoid molecular signature upon leukaemic transformation, while adult leukaemic cells show higher expression of myeloid proteins. Finally, we describe the proteomic composition of the extracellular environment of fetal and adult hematopoietic progenitors at steady state and integrate this data with the cellular proteomic data of leukemia initiation. This reveals ligand-receptor pairs with potential regulatory interactions and highlight differential regulation of Igf2 bioavailability upon initiation of fetal- and adult-origin leukemia. Summary/Conclusion: Our study provides a comprehensive resource of ontogeny- and malignancy-driven changes in the intra- and extracellular proteome of haematopoietic progenitor cells, which in combination with the results from our functional assays sheds new light on age-specific targetable pathways in MLLr leukaemia. Keywords: Acute leukemia, Proteomics, MLL